The following account of the prior art relates to one of the areas of application of the present invention, distributed temperature measurement based on optical Raman backscattering. The present invention may, however, be applied to other measurement systems that are not based on optical backscattering. An example of a prior art FMCW backscattering system to which the present invention would be relevant is described in EP-1 548 416.
An optical backscattering system may e.g. comprise a modulated laser source, a sensor for capturing a spatially distributed measurement of a physical quantity (e.g. temperature, force, humidity, etc.) in the form of an optical waveguide, e.g. an optical fibre, mixing, filtering and receiving elements (including opto-electronic converters), signal processing and calculation units for transforming and evaluating the backscattered signal(s) and for determining the spatially distributed profile of the physical quantity in question.
A typical problem for an optical back scattering system is to provide an unambiguous calibration of the measurement system (including the sensor). A prior art FMCW (Frequency Modulated Continuous Wave) system for measuring a distributed temperature profile, for example an optical frequency domain reflectometry system, as e.g. described in EP-0 692 705, has to be manually calibrated in that the sensor (e.g. an optical fibre) is exposed to a number of well defined hot-spots at predetermined locations along its length for which the actual temperatures at those locations are measured with calibrated temperature sensors so that an actual profile is known.
Due to the properties of the complex frequency data (i.e. data comprise real and imaginary parts); the values of the corrected frequency data are ambiguous.
Other problems with prior art systems include contributions to measurement errors due to one or more of the following:
a) DC-Errors:
a1) The FMCW temperature method is e.g. based on the measurement of Raman backscattering of light from an optical fibre as a function of the laser frequency modulation (fm). The backscattering curves of the Raman light as function of the fibre length are based on the calculation of the Inverse Fourier Transformation of an electrical photo detector signal. The algorithm of this Inverse Fourier Transformation requires the complex measurement of the backscattering signal between fm=0 Hz and the maximal laser modulation frequency. The measurement of the first frequency point (DC value) for the Inverse Fourier Transformation is difficult, because this value is superimposed with the classic steady component of the photo detector signal.a2) The DC value is not a constant. The value is dependent on the sensor properties (e.g. different lengths or different specifications of optical waveguides used as sensor, cf. FIGS. 6 and 7 and the corresponding description).b) Errors Due to Tolerances and Nonlinear Behaviour of Components.
The FMCW Raman techniques require the fibre measurement of a very weak Raman backscattering light signal (down to the pico watt range) as a function of frequency modulated laser light over a broad frequency band (fm may e.g. be in the range from 0 Hz up to 100 MHz). The average of the intensity of the laser light is constant. Due to the weak detector signal the tolerances of optic components (laser, photo detector, filter, etc.) and electronic components (amplifier, mixer, filter, etc.) have a perceptible impact on the quality of the backscattering and resulting temperature curves. Likewise, the nonlinear behaviour of optic and electronic components produces contortion in the frequency data. The result is a nonlinear contortion along the temperature profile which reduces the accuracy of the temperature measurement system.
c) Errors Due to Cross Talk Between Different Measurement Channels.
Cross talk between different measurement channels may cause an additional error in the form of random noise and nonlinear interferences in the temperature profile (cf. e.g. FIG. 10 and the corresponding description).
d) Errors Due to Ageing Effects.
Aging effects on optical and electronic components also have an impact on the quality and stability of the measurement devices due to the weak Raman detector signal and the above mentioned FMCW measurement dependence between frequency signal and temperature profile.
e) Errors Caused by a Change of the Sensor Line.
In case of a change of the optical sensor line, a preceding calibration may not be valid anymore. This is mainly caused by an impact on the DC value of the frequency data.
The above mentioned undesirable effects all have an impact on the quality and the stability of a Raman temperature measurement system. The elimination and separation of these effects is not possible with current calibration procedures. As a consequence, calibration procedures must be repeated when alteration of system qualities are noticed. This is time consuming and cost intensive. An additional weakness of current calibration procedures is the strong impact of incurred errors on the resulting measurement quality (e.g. system accuracy (precision)). This means that new applications having higher requirements to measurement precision are excluded with current systems.